11
presented with Γo. If the reflec-
tion coefficient of the matching
network is other than Γo, then the
noise figure of the device will be
greater than Fmin based on the
following equation.
Typically for FETs, the higher Γo
usually infers that an impedance
much higher than 50Ω is required
for the device to produce Fmin. At
VHF frequencies and even lower
L Band frequencies, the required
impedance can be in the vicinity
of several thousand ohms.
Matching to such a high imped-
ance requires very hi-Q compo-
nents in order to minimize circuit
losses. As an example at 900 MHz,
when airwwound coils (Q>100)
are used for matching networks,
the loss can still be up to 0.25 dB
which will add directly to the
Noise Parameter
Applications Information
Fmin values at 2 GHz and higher
are based on measurements while
the Fmins below 2 GHz have been
extrapolated. The Fmin values are
based on a set of 16 noise figure
measurements made at 16
different impedances using an
ATN NP5 test system. From these
measurements, a true Fmin is
calculated. Fmin represents the
true minimum noise figure of the
device when the device is pre-
sented with an impedance
matching network that trans-
forms the source impedance,
typically 50Ω, to an impedance
represented by the reflection
coefficient Γo. The designer must
design a matching network that
will present Γo to the device with
minimal associated circuit losses.
The noise figure of the completed
amplifier is equal to the noise
figure of the device plus the
NF = Fmin + 4 Rn
|Γs – Γo | 2
Zo (|1 + Γo|2)(1–Γs|2)
Where Rn/Zo is the normalized
noise resistance, Γo is the opti-
mum reflection coefficient
required to produce Fmin and Γs is
the reflection coefficient of the
source impedance actually
presented to the device. The
losses of the matching networks
are non-zero and they will also
add to the noise figure of the
device creating a higher amplifier
noise figure. The losses of the
matching networks are related to
the Q of the components and
associated printed circuit board
loss. Γo is typically fairly low at
higher frequencies and increases
as frequency is lowered. Larger
gate width devices will typically
have a lower Γo as compared to
narrower gate width devices.
noise figure of the device. Using
muiltilayer molded inductors with
Qs in the 30 to 50 range results in
additional loss over the airwound
coil. Losses as high as 0.5 dB or
greater add to the typical 0.15 dB
F
min of the device creating an
amplifier noise figure of nearly
0.65 dB. A discussion concerning
calculated and measured circuit
losses and their effect on ampli-
fier noise figure is covered in
Agilent Application 1085.
losses of the matching network
preceding the device. The noise
figure of the device is equal to
Fmin only when the device is
HP [ HEWLETT-PACKARD ]
